1. Field of the Invention
The present invention generally relates to an image forming apparatus such as a printer, a copying machine, and a facsimile, and more particularly to a displacement detector and a mark detector which are capable of increasing accuracy and stability of a rotation or displacement of a rotating member, such as a photo conductor belt, a transfer belt, a paper conveyance belt, a photo conductor drum, a transfer drum, etc.
2. Discussion of the Background
A background image forming apparatus such as a printer, a copying machine, and a facsimile includes a rotating member, such as a photo conductor belt, a transfer belt, a paper conveyance belt, a photo conductor drum, a transfer drum. In the image forming apparatus, to realize high accuracy positioning of an image on a transfer member which is a rotating member or a conveyed member, a rotation or displacement of the transfer member should be controlled with high accuracy. But it is difficult to maintain high accuracy in positioning an image, because the amount of rotation of the rotating member varies easily in many reasons. Especially in a color image forming apparatus, a variation of a rotation of a rotating member causes a displacement between the colors, so that a position of the image in each color does not overlap correctly.
To reduce a positioning error of a rotating member, in one example, a rotary encoder is provided on the axis of a rotating member to indirectly measure a surface velocity of the rotating member. A speed of a driving motor is controlled according to the measured speed of the rotating member. However, it is difficult to maintain high accuracy in the rotation of the rotating member, because a decentering of the rotating member easily causes a variation of a surface velocity of the rotating member, even though the driving motor is controlled according to the detected speed of the rotation of the rotating member.
When an endless belt is used as a photo conductor belt or a middle transfer belt, variation in the thickness of the belt, decentering of a roller for rotating the belt, or a variation in the speed of a driving motor for conveying the belt causes a speed variation of the belt. Especially in a color image forming apparatus, a positioning error by the speed variation of the belt forms a waveform which has two or more frequency components during one rotation of the belt as shown in FIG. 1. FIG. 1 is a graph showing relations of time and a positioning error. The speed variation of the belt causes a displacement between the colors, such as cyan (C), magenta (M), yellow (Y), black (K), so that a position of the image in each color does not overlap correctly. Therefore, a speed variation of a belt causes lower image quality, such as bad positioning between colors, difference of a color, or other undesirable qualities.
To reduce speed variation of a belt, in one example, a rotary encoder is provided on the axis of a rotating member for driving the belt to calculate the amount of the rotation and an average speed of the rotating member using output pulses of the rotary encoder. Driving motor speed is controlled according to the calculated results. However, it is difficult to maintain high accuracy in the rotation of the rotating member because the number of the rotations and the average speed of the rotating member is calculated and obtained indirectly.
To detect the number of the rotations of the rotating member directly, in one example, a reflective mark is provided on a surface of a belt. A belt conveyance unit controls the rotation of the belt with a feedback control calculating belt surface speed by using binary pulse intervals which is acquired by sensing the reflective marks.
However, a belt used as a rotating member for image forming is easily deformable and has a deviation in thickness. Therefore, when the belt is rotating, a variation of gap or angle between a mark on the belt and a sensor for detecting the mark may occur. This variation may cause variation in the amount of light accepted by the sensor. FIG. 2 is a graph showing relations of time and a mark detection signal, and time and a binary signal of the mark detection signal. As shown in FIG. 2, the mark detection signal with the sensor may have a variation in amplitude. A binary signal, which is acquired by a comparison of the amplitude limit with a reference voltage 0 V and the mark detection signal, may have a variation in its pulse intervals. Even if a high pass filter is used to remove offset signals caused by fluctuation in the rotating member speed, the electric signal may not be detected due to decline in the electric signal because the speed may not be within the pass band of the high pass filter.